cmd/builder: add flag for whether to build subrepos
cmd/builder: add flag for whether to build subrepos

Change-Id: I814c7770c0ad620c85b8e8f5fcf50420a7ce95c4
Reviewed-on: https://go-review.googlesource.com/25191
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build